1. Field of the Invention
The present invention relates to a power cord with plug for use in an electric appliance, and more specifically to a less expensive power cord with a safety device for detecting damage of a power line in the cord at an earlier stage, shutting off the power, and guaranteeing the safety of the electric appliance and its environment.
2. Description of the Related Art
There has been a conventional power cord with plug for use in supplying power to an electric appliance from a commercial power receptacle. Since the power cord is normally bent and pulled each time it is used, the repeated bendings and pulls cause fatigue with the passage of time especially at the base portion of the plug or the electric appliance, thereby often damaging the cord at these portions.
Conventionally, to prevent the damage of the cord caused by the bendings and pulls, there have been no effective methods other than thickening a power line or a sheath. However, these methods only protect a cord against bendings and pulls, but lose the flexibility of the cord, and are poor in ease-of-use. In this situation, to improve the protection of power cords against bendings and pulls, there are a number of cases in which a power cord is reinforced by providing a cord protector for the power cord at the base portion of the plug or the electric appliance.
FIG. 1 shows a power cord provided with the above mentioned cord protector. As shown in FIG. 1, the cord contact portion of a power cord 1 is covered with an insulating sheath material 2, and contains two power lines 3 arranged in parallel. One end of the cord (left end in FIG. 1) is fixed to a plug 4, and the two power lines 3 are connected to two respective plug terminals 5 of the plug 4. A cord protector 6 is attached to the base portion of the plug 4. Thus, the base portion of the plug to which strong bending and pulling force is applied can be reinforced, thereby successfully generating a durable code.
In an example often reported in the U.S., a hair dryer is carelessly dropped into a bathtub, and an electric shock accident occurs in the bathtub. Therefore, to prevent the electric shock accident, it has been made mandatory to attach a ground-fault interrupter to a plug. The ground-fault interrupter can either shut off the power by detecting a ground-fault current or shut off the power by providing a detection line for detecting a current flowing through it. The ground-fault interrupter which detects a ground-fault current can be applied when a bathtub is insulated, and the ground-fault interrupter which is provided with the detection line can be applied when a bathtub is not insulated.
However, although a power cord is effectively reinforced by any means as by the above mentioned cord protector, the inside power lines are broken with fatigue with the passage of time with years of use. However, since the power lines of a power cord are covered with a sheath material, and are safely maintained by the sheath material, they are not completely broken and are in a half-broken state in many cases. Thus, if the power cord is used with the power lines maintained in the half-broken state, then the electrical resistance of the half-broken portion becomes high, and a large amount of heat is locally generated at this portion, thereby often causing the danger of a fire.
Furthermore, when a heater appliance such as a hair dryer, an electric ironing device, etc. is constantly used in hand, the power cord in use is subject to a heavy load. Therefore, the power cord is more seriously damaged and more easily broken than the power cords of other types of appliances. However, since the heater appliances consume larger power, a large amount of heat is generated in the above mentioned half-broken state, thereby easily causing the danger of a fire.
The ground-fault interrupter for preventing an electric shock accident in a bathtub as in the example in the U.S. has been developed to shut off the power if a leakage occurs when a user mishandles the appliance or if the appliance becomes wet by a careless user. Therefore, it does not prevent the danger caused by other factors, for example, an internal break, etc. from fatigue with the passage of time, etc.
The present invention has been developed to solve the above mentioned problems with the conventional technology, and aims at providing a less expensive power cord with a safety device for detecting the damage of the power lines in a cord at an earlier stage, shutting off the power, and guaranteeing the safety of an electric appliance and its environment.
The power cord with a safety device according to the present invention is a power cord for supplying the power from a commercial power receptacle to an electric appliance, its cord contact portion is covered with an insulating sheath material, the cord contains two power lines and a detection line, a plug includes a power shutdown device for operating upon detection of the current flowing through the detection line. The detection line is provided between two power lines arranged in parallel, and the sheath portions between each of the two power lines and the detection line is thinner than the sheath portion of the cord contact portion.
The above mentioned detection line is covered with the sheath material having a lower melting point than the sheath material of the power line, and covered with a conductive sheath material.
Thus, when the power line is damaged by the fatigue with the passage of time by bendings and pulls in use, etc., the detection line and the power line more easily contact each other and make a short-circuit, thereby successfully preventing the local generation of a large amount of heat by an electric shock from a damaged portion and by use without knowing the damage. Furthermore, the occurrence of a disaster such as a fire, etc. can be avoided.
The power cord with a safety device can be configured by two power lines arranged in parallel; an insulating internal sheath layer covering the power lines; a conductive sheath material provided around the internal sheath layer; an insulating external sheath layer forming a cord contact portion by covering the conductive sheath material; and a plug containing a power shutdown device operating upon detection of a current flowing through the conductive sheath material.
The power shutdown device can also be configured by, for example, a leakage detecting power shutdown device, or, for example, a wet detecting power shutdown device.
With the above mentioned configuration according to the present invention, since the damaged portion of a power line contacts the detection line when internal damage occurs before the cord contact portion is damaged, the failure by the damage of the sheath of the cord contact portion of the power line, which cannot be avoided by any means other than a conventional protector, can be successfully avoided, thereby successfully avoiding a serious failure in the environment caused by a combined effect of a leakage, an electric shock, a burn, a fire, etc. occurring by the deterioration with the passage of time as well as by careless of mistaken handling. As a result, a safe and reliable power cord can be provided.